So you have a 'running" 2bbl system and you want to put it on a 4bbl 454 just to get the motor-home moving.
Napa or Oriylies for a 2bbl to 4bbl adapter (or a 4bbl to 2bbl adapter upside down), extra gaskets and goop to get it sealed and pop in on. Make sure the cooling system is spot on, run a forklift tank strapped somewhere convenient, ask around to borrow a extra tank or two. Pace Propane in Redmond can order the HP hose pretty quickly, or you can buy it on-line. I'm sure there are other propane places that can get it. There is a couple forklift shop in Sodo but I have never been there to see if they are parts friendly. First one I'm thinking of is the forklift wrecking yard off Airport at Spokane st.
When your all done, I have a couple extra 4bbl mixers I might be willing to trade one for your 2bbl set up. No idea on the condition, have never run them, and I think they are square bore, but those adapters are easy to find. Would save me changing manifold for one of my trucks that sits alot and I would like to put on lpg.